I propose to investigate the mechanism that underlies the prolonged depolarizing afterpotentials (PDAs) present in many invertebrate photoreceptors. The questions I seek to answer are whether the PDA, like the light coincident receptor potential (LCRP), is a summation of bumps and, if so, whether the bumps that constitute the PDA are different from those that make up the LCRP. I will also compare PDA bumps and LCRP bumps with those induced by chemical agents such as fluoride and non-hydrolyzable GTP analogs, and investigate the interaction of the different types of bumps with one another. I intend further to describe the post-stimulus timecourse of the PDA in terms of its underlying bump processes. A number of PDA-defective mutants of Drosophila have been previously isolated, and I will analyze them, as well as wild-type Drosophila and Musca, in the terms outlined above. In this way I hope to define the sequence of events leading to bump generation during PDA or LCRP, and which step or steps in this sequence have been affected in each mutant. It is my intention to synthesize these findings in the form of a quantitative model of the phototransduction process.